This invention relates to polishing pads for polishing such materials as semiconductor silicon wafers to a high degree of smoothness.
With the miniaturization of integrated circuits (LSI), the surfaces of semiconductor silicon wafers are generally polished to a high degree of smoothness by chemical mechanical polishing (CMP). This chemical mechanical polishing is carried out using a surface polishing apparatus, for instance as shown in FIG. 13. The surface polishing apparatus has a rotary table 2 with a polishing pad 1 spread and fixed thereupon, a top ring installed over the rotary table 2 and movable vertically to hold a work piece 3 to be polished (such as a semiconductor silicon wafer), and a polishing solution nozzle 6 to supply a polishing solution (slurry) 5, with abrasive grains such as SiO.sub.2 and Al.sub.2 O.sub.3 suspended therein, onto the polishing pad 1. The apparatus works in this manner. Rotary table 2 and top ring 4 rotate independently of one another, with polishing solution 5 being supplied onto polishing pad 1 from polishing solution nozzle 6 and with top ring 4 pressing the surface to be polished, or the lower surface 3a, of the work piece 3 against the upper surface, or the polishing surface 1a, of the polishing pad 1. In this way, the work piece surface 3a is polished to a high degree of smoothness (specular surface), with the polishing solution 5 placed between the work piece surface 3a to be polished and the polishing pad surface 1a.
Among examples of polishing pads are one formed of nonwoven fabric with randomly arranged polyester fibers partially impregnated and hardened with polyurethane resin and another one made of a foam structured sheet of the urethane type such as a foam structured (porous) polyurethane sheet. Those kinds of pads, porous in structure with many fine pores scattered in the polishing pad surface, exhibit excellent polishing characteristics. Those pores are responsible for increased retention of the polishing solution on the polishing pad surface. The pores are also to work to keep the work piece from sticking to the polishing pad surface.
The problem with those prior art porous structured polishing pads is that the pores formed in the surface layer of the pad 1 including the polishing pad surface 1a are varied in size and irregular in position and arrangement (pore-arrangement pattern, i.e. positional relation between the pores). That contributes to a lowered polishing rate, increased non-uniformity in degree of polishing in a given work piece to be polished (the surface of a silicon wafer), faulty polishing or a damaged surface, and the like. Because of such non-uniform polishing performance, a good polished surface is difficult to obtain.
To illustrate further, the area 1b of the polishing pad surface 1a with which the work piece surface 3a to be polished comes into contact (work piece contact area 1b) moves as the rotary table 2 and the top ring 4 rotate. If the polishing pad surface is irregular in pore size and pore-arrangement pattern, the work piece contact area 1b of the polishing pad surface 1a will change in pore formation (number of pores, size, pattern, etc.) as it moves. Furthermore, the pore formation of the polishing pad surface 1a changes constantly in accordance with decrease in thickness of the pad as the polishing goes on. For example, as the polishing pad surface wears in the course of the polishing process, some shallow pores will disappear while other pores under the surface will come out. In this way, the polishing pad surface 1a changes in pore number, size, and pattern. Also, a polishing pad surface studded with shallow pores (including pores made shallow by wearing of the pad surface) could cause faulty polishing (resulting in a damaged polished surface) because abrasive grains and/or polishing dust, detained and accumulated in the shallow pores, could shave the polishing work piece surface 3a locally deep.
Thus prior art polishing pads formed of nonwoven fabrics, urethane-type foam-structured sheets, etc. have presented the problem that they would change in polishing characteristics of the pad-work piece contact area 1b, such as retention of polishing solution, polishing rate, and non-uniformity, during the course of the polishing process.